Cancel animation when a custom Timeline is used (facebook#35567)

Follow up to facebook#35559.

The clean up function of the custom timeline doesn't necessarily clean
up the animation. Just the timeline's internal state.

This affects Firefox which doesn't support ScrollTimeline so uses the
polyfill's custom timeline.

Author: sebmarkbage

fixtures/view-transition/src/components/Page.js

@@ -150,6 +150,8 @@ export default function Page({url, navigate}) {
       const cleanup1 = timeline.animate(animation1);
       const cleanup2 = timeline.animate(animation2);
       return () => {
+        animation1.cancel();
+        animation2.cancel();
         cleanup1();
         cleanup2();
       };

packages/react-dom-bindings/src/client/ReactFiberConfigDOM.js

@@ -2525,6 +2525,7 @@ function animateGesture(
       delay: reverse ? rangeEnd : rangeStart,
       duration: reverse ? rangeStart - rangeEnd : rangeEnd - rangeStart,
     });
+    viewTransitionAnimations.push(animation);
     // Let the custom timeline take control of driving the animation.
     const cleanup = timeline.animate(animation);
     if (cleanup) {